A single crystal, also called monocrystal, is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. Single crystals are used in many applications, such as scintillators in nuclear medicine imaging including PET and SPECT detectors, in addition to other fields such as semiconductor manufacturing, lasers, telescopes, turbines, etc.
Fabrication of single crystals usually involves the growing of a crystal layer by layer. Techniques to produce large single crystals (boules) include slowly drawing a rotating “seed crystal” in a molten bath of feeder material. A seed crystal is a small piece of single crystal material from which a large crystal of usually the same material is to be grown.
One commonly used method of growing single crystals is the Czochralski (CZ) process. The CZ process is often used to grow large cylindrical boules. In the CZ process, a starting material, such as silicon, sodium iodide, etc., is heated in a crucible to produce a melt. A seed crystal is slowly lowered over the melt so that the seed crystal itself may begin to melt slightly.
The seed crystal is then gently brought into contact with the melt. The temperature of the melt is lowered to cause the melt to freeze onto the seed crystal. As this occurs, the seed crystal is slowly pulled out of the melt and is rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single crystal, cylindrical ingot from the melt. When the crystal reaches its predetermined length, the rate of pulling is increased to pull the crystal away from the melt to prevent further growth. The grown crystal is then cooled to be used for its designated purpose.
A crystal grown by the CZ method is highly susceptible to dislocations. A dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of real materials.
Some causes of dislocations in single crystals include dislocations in the seed crystal and thermal shock, caused by rapid cooling of the crystal as it is removed from the melt.
Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress overcomes the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the crystal's structure to fail.
Dash necking is one method used to help prevent thermal shock. In dash necking, the seed crystal is rapidly pulled from the melt in the beginning of the process. This creates a thin portion, or neck, of crystal that is virtually dislocation free. However, since the neck is thin it is subject to cracking especially when it is used to support a large single crystal.
A second method used to help prevent thermal shock is to chill the shaft that holds the seed crystal. The shaft controls the dissipation of heat from the single crystal to better control thermal shock. However, there is a possibility that the seed crystal will slip out of the holder. The seed crystals are often brittle and, therefore, it may be difficult to tightly hold the seed crystal without damaging it.
In order to overcome the problem of holding the seed crystal and to prevent damage during assembly, a cushioning material is often placed between the seed crystal and the holder. However, the cushioning material itself is heat-insulating, which undesirably limits the heat-transferability of the cooling shaft.